The casting of alloys may be done by any number of methods known to those skilled in the art, such as direct chill casting (DC), electromagnetic casting (EMC), horizontal direct chill casting (HDC), hot top casting, continuous casting, semi-continuous casting, die casting, roll casting, and sand casting.
Each of these casting methods mentioned above has a set of its own inherent problems, but with each technique, surface imperfections can be an issue. In the aluminum alloy casting art, molten metal (or melt for brevity) surface oxidation can produce various surface imperfections in cast ingots such as pits, vertical folds, oxide patches and the like, which can develop into cracks during casting or in later processing. A crack in an ingot or slab propagates during subsequent rolling, for example, leading to expensive remedial rework or scrapping of the cracked material. One mechanical means of removing surface imperfections from an aluminum alloy ingot is scalping. Scalping involves the machining off of a surface layer along the rolling faces of an ingot after it has solidified. However, scalping results in lost metal.
Rectangular ingot yields for high magnesium alloys, such as 7050 and other 7xxx alloys as well as 5182 and 5083 alloys are especially prone to surface defects and cracking caused by initiation at vertical folds on the surface of the ingot. In the past, beryllium has been added, usually at part per million (ppm) levels to some of these alloys to control melt surface defects, and to prevent magnesium loss due to oxidation. In addition, materials, especially those containing fluorine, such as boron trifluoride and ammonium fluoroborate, have been used to promote uniform oxide distribution and therefore reduce surface defects and cracking. However, the use of these additives can be very costly and beryllium itself may fall into disuse due to allegations regarding health, disposal, and environmental issues that it creates. Furthermore, the use of gases can create toxic and corrosive gaseous atmospheres. For these reasons, suitable replacement strategies to control the nature of oxides during casting are needed.
In the casting of aluminum alloys it is also known in the art to use a mold lubricant. Satisfactory ingot surfaces can be obtained using a lubricant that is effective in keeping aluminum from sticking to the mold at high temperatures used in casting aluminum alloys. In early casting practices, greases were commonly employed as mold lubricants. However, with the advent of modem casting methods, including continuous or semi-continuous casting, free flowing oils have been used to provide continuous lubrication and have replaced the use of greases as mold lubricants.
Continuous casting refers to the uninterrupted formation of a cast body or ingot. For example, the body or ingot may be cast on or between belts, as in belt casting; between blocks, as in block casting; or in a mold or die that is open at both ends, as in direct chill (DC) casting. Casting may continue indefinitely if the cast body is subsequently cut into desired lengths. Alternately, the pouring operation may be started and stopped when an ingot of desired length is obtained. The latter situation is referred to as semi-continuous casting.
Continuous lubrication is required for fully continuous casting and offers a number of advantages for semi-continuous casting. These advantages include substantial reduction of flame and smoke, substantial reduction of dragging and tearing tendencies near the end of the cast, and allowing casting practices that produce better quality and more uniform surfaces.
Despite the use of continuous lubrication during casting, a limitation of current ingot casting practice exists in the non-uniform growth of oxide at the meniscus of molten metal at the mold interface. Non-uniform oxide growth at the meniscus of the molten metal and mold interface is particularly problematic for alloying elements that rapidly oxidize in air or in air containing moisture. For example, alloys containing lithium and magnesium may oxidize rapidly and in both cases, the vapor pressure of the element is higher than that of aluminum. As a result, magnesium and lithium may diffuse to the surface of the ingot and react with oxygen or moisture in the ambient air.
Distribution of the surface oxide on the ingot head and at the meniscus plays an important role in fold prevention or formation. Data from previous research shows that humid air can produce an oxide/hydroxide film that protects magnesium-containing alloys from runaway or uncontrolled magnesium oxidation at molten metal temperatures. Since the weight gain of the magnesium-containing alloy is significantly reduced in humid air as compared to dry air, the oxide layer is thinner and the oxide distribution is believed to be more uniform. Another mechanism that plays a part in the transformation of molten metal to solid metal is the heat transfer at the mold wall between the molten metal and lubricant coated mold wall.
In addition, controlling the stability of the meniscus during casting also plays an important role in fold prevention and formation. An unstable meniscus is often observed during casting by the change of the meniscus shape running along and around the mold face. It is believed that increasing the viscosity of the casting lubricant will provide less fluidity and thus greater stability of the molten metal meniscus.
There remains a need for an effective alternative to beryllium and fluorine containing materials to prevent surface imperfections, such as vertical folds, pits, oxide patches and the like from forming during aluminum ingot casting, and to control the nature and distribution of oxides, particularly when casting aluminum that is alloyed with elements like magnesium and lithium. Such an invention would be instrumental in preventing cracks, which can form during casting or can develop in later processing. Finally, the invention preferably would have no adverse affect on alloy properties.
The primary object of the present invention is to provide a lubricant composition that allows for uniform distribution of surface oxide at the meniscus formed between the molten aluminum and the mold during the continuous and semi-continuous casting of aluminum alloy ingots.
Another object of the present invention is to provide a lubricant composition that promotes a uniform and controlled rate of heat transfer at the interface formed between the molten aluminum and the mold during the continuous or semi-continuous casting of aluminum alloy ingots.
A still further object of this invention is to provide a casting lubricant that promotes uniform oxide distribution without requiring the use of toxic and corrosive gaseous atmospheres, and thus eliminating associated emissions and equipment corrosion.
Still another object of this invention is to provide a method that promotes uniform oxide distribution on aluminum alloy ingots or castings that does not require beryllium additions to the alloy and fluorine containing atmospheres.
A final object of the invention is to provide a casting lubricant composition that allows for a stable meniscus during the continuous and semi-continuous casting of aluminum alloy ingots.
These and other objects and advantages are met or exceeded by the instant invention, and will become more fully understood and appreciated with reference to the following description.